Telescope Optics Formulas with Calculators
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List of Calculators |
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1. Focal Ratio 2. Focal Length |
4. Dawes Limit |
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Hi, Ray Shore here. This page is dedicated to formulas related to telescope optics. I have listed the main formulas below for these calculations; each with a handy calculator. To the right, I have listed the specifications for my Schmidt-Cassegrain optical tube. The examples below will reference these specs. |
Specifications for My C8 Optical Tube:
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Focal Length/Aperture
The smaller the f-ratio, the faster the telescope. Fast telescopes (e.g. f/4 or f/6) provide wider fields of view (FOV) making them most suitable for deep space observation.The larger the f-ratio, the slower the telescope. The FOV becomes smaller as the f-ratio gets larger. Telescopes with large f-ratio's (e.g. f/8 or f/10) are most suitable for high power planetary work.
Example: my telescope has a large f-ratio (f/10). To calculate, I take the focal length of 2032mm and divide by it's aperture of 203.2mm. The result is f/10. My scope is well suited for high planetary observation and imaging.
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Important note: the focal length and aperture should be in the same units for this calculation. In my example above, I converted my 8" aperture to 203.2mm because the focal length was in mm. |
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Back to Top Aperture X Focal Ratio Description: length of the light path from
the lens or mirror to the focus point. Example: I take my aperture of 203.2mm and
multiply by my f-ratio of f/10. The result is 2032mm. Focal Length Calculator Note: some people prefer that their focal length be stated in inches rather than millimeters. Click here to convert to inches Telescope Focal Length / Eyepiece
Focal Length Example: If I'm using a 25mm eyepiece with my scope, I take my scope focal length of 2032mm and divide by 25mm and I get 81X of magnification. A general rule of thumb for applying power is no
more than 50X of magnification per inch of aperture. Example: the aperture of my scope is 8". Taking
that times 50 gives me a limit of about 450X. Generally I don't come close to
using that much power! Magnification Calculator Solar System Software Tools 4.56 arc seconds/ objective diameter in inches Pertains to the resolving
power of an optical system (generally referred to when splitting double stars).
The higher the resolving power, the smaller the minimum visible detail or
minimum angle of resolution. The lower the resolving power, the bigger the
minimum visible detail or minimum angle of resolution. Example: taking 4.56 arc
seconds and dividing by my scopes objective diameter of 8" gives me 0.57 arc/sec of
resolving power. This means that I could theoretically split a double star with
0.57 arc/seconds of separation. Dawes Limit Calculator Click here to convert from millimeters to inches Apparent Field of Eyepiece / Magnification This is important if you want
to know the size of the object that can be observed. The apparent field of an
eyepiece should be supplied by the eyepiece manufacturer. Example: if I'm using a
10mm eyepiece with an apparent field of view of 46 degrees then I divide 46
by 203.2 (power derived from 2032/10). This gives me a true field of view of 0.2
degrees. True FOV Calculator
